Method and article for providing an indication of the presence of air in steam

ABSTRACT

A method and apparatus for evaluating the efficacy of prevacuum steam sterilizers by separating, collecting and detecting the presence of air in association with steam. A receptacle containing an adsorbent directly separates air from steam without changing the physical state of the steam. The apparatus has a portal communicating with the sterilizing environment and a defined path for the steam to traverse from the portal to a distal end of the apparatus. The distal end of the apparatus is attached to an air collection chamber which contains an air indicator. The indicator can be chemical or biological, and indicated whether any air pockets existed in the steam.

This is a continuation of application Ser. No. 08/013,147 filed on Feb.1, 1993 now U.S. Pat. No. 5,270,217.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices for evaluating the efficacy ofsteam sterilizers, and, more specifically, to devices that provide anindication of the presence of air in a sterilizer during thesterilization process.

2. Description of the Prior Art

Steam sterilization of an object is accomplished by placing steam incontact with the object. The object is cooler than the steam. Thus, thelatent heat of the steam is transferred to the object. The latent heatof steam, which is 540 calories per gram makes steam particularlyattractive as a sterilizing medium. For example, use of dry air at 320°F. requires exposure for 60 minutes to achieve sterilization equivalentto that obtained by exposure for approximately 15 minutes to saturatedsteam at 250° F.

To achieve effective steam sterilization, the steam should be fullysaturated, that is, without the presence of contaminants such as air. Ifair is present within the steam, repeated collapse of the steam againstthe cooler surface of the object to be sterilized will occur, causingaccumulation of air and the formation of an air pocket. Air pocketsshield microorganisms on the object from the steam, and preventstransfer of a sufficient quantity of heat to the object, resulting inineffective sterilization. Accordingly, prevacuum steam sterilizersremove air from the sterilization chamber prior to the introduction ofpressurized vapor, or steam, into the chamber during the sterilizationcycle. However, an ineffective prevacuum cycle can leave air in thechamber, which will remain there and accumulate during the sterilizationcycle. Also, air can be introduced into the chamber as a result ofequipment leaks and malfunctions, and through the steam supply.

To detect the presence of air during steam sterilization processes, Dr.J. Bowie and Mr. J. Dick described in 1963 a test pack that monitorsprevacuum sterilization. The test pack consisted of 29 huckaback towelsfolded into fourths along their lengths and doubled across their widthsto give eight thicknesses of cloth. Each folded towel was stacked, andsteam sensitive autoclave tape, formed to resemble a St. Andrew's cross,was placed at various levels in the pack. The pack was placed within adressing casket or overwrapped in fabric, and placed in thesterilization chamber. After the evacuation cycle had been conducted,steam was introduced into the chamber, where it entered the test packand collapsed to water as it transferred its latent heat to the coolertowels. Any air that was present in the steam accumulated as the steamcollapsed. The air was forced by the pressure within the sterilizertoward the density center of the test pack, where it formed pockets. Anyair remaining in the test pack after the evacuation cycle also would beforced by the steam to the density center of the pack. The autoclavetape, through its steam indicator ink, detected the resulting airpockets. A uniform change in color of the steam sensitive indicator inkon the tape, from white to black, indicated completion of a successfulsterilization cycle. However, an incomplete color change of the inkindicated an ineffective sterilization cycle. Accordingly, any air thatmigrated to the indicator ink located at the density centers of the testpack prevented steam from making contact with the entire indicator and,thus, prevented the ink from undergoing a uniform color change. TheBowie-Dick test is discussed further in The Lancet, Mar. 16, 1963, pp.586-587. The Bowie-Dick protocol is further described in the NationalStandards and Recommended Practices for Sterilization: RecommendedPractice--Good Hospital Practice--Steam Sterilization and SterilityAssurance, 6.7 (1988), published by the Association for the Advancementof Medical Instrumentation, 3330 Washington Boulevard, Arlington, Va.22201.

The Bowie and Dick test pack performed several functions. First, andmost basically, it separated steam from any air present in the steam,since the towels functioned like a heat sink, and included an indicatorthat indicated contact with steam. Importantly, it provided a"challenge" to the removal of air during the evacuation cycle. That is,it was more difficult to remove air from the test pack than it was toremove air from the objects that were to be sterilized, due to thenatural entrapment of air by the interstices of the towels forming thetest pack. Thus, the absence of air pockets from the test pack duringsterilization justified the conclusion that air pockets were not presentanywhere in the chamber during the sterilization process. Similarly, thetest pack provided a "challenge" to the entry of steam into the testpack during the sterilization cycle, justifying the conclusion thatsteam had adequately contacted the articles undergoing sterilization ifthe indicator ink on the tape in the pack changed color uniformly.Finally, the towels of the test pack adsorbed the water created by thecollapsing steam, thus preventing it from travelling to the autoclavetape where it could revaporize and contribute to a false indication. Insummary, then, the Bowie and Dick test pack offered (i) a challenge toremoval of air from the test pack, (ii) a heat sink that absorbed thelatent heat of steam and separated air and steam, (iii) resistance topenetration by steam into the pack, (iv) an indicator that detected, thepresence of air and (v) an absorbent that retained the water generatedby the change of state of the steam.

Nonetheless, Bowie and Dick type testing proved less than satisfactorydue to the need for cotton towels, varying directions by manufacturersin the use of their indicator sheets, variations in folding techniques,non-uniform towel conditions, variations in the outer wrapping materialutilized, and the presence of laundry chemicals on the towel, whichcould react with the indicator. Therefore, workers in the art attemptedto develop devices for indicating the presence of air duringsterilization that afforded more uniformity in use, and more reliableresults, and that provided the benefits of the Bowie-Dick test pack.

Joslyn U.S. Pat. No. 4,115,068 discloses an air indicating device foruse in a steam or gas sterilizer comprising an upright, insulated tubewith a closed top and open bottom. The upper portion of the tubecontains a thermal indicator surrounded by a heat sink material. Steamenters the tube through the bottom and is forced toward the top of thetube. The steam that contacts the heat sink will be condensed; however,the steam that contacts the indicator strip will transfer its heat andhumidity to the strip. As the temperature of the heat sink rises duringthe sterilization cycle, it can revaporize the condensed steam, whichwill provide a false indication of the presence of saturated steam.Additionally, the tube does not offer a challenge to air removal duringthe evacuation cycle, and it does not offer resistance to steampenetration during the sterilization cycle.

Augurt U.S. Pat. No. 4,486,387 discloses a disposable test pack fordetermining the effectiveness of a prevacuum steam sterilization cycle.The pack consists of a steam indicator sheet placed between stacks ofporous paper sheets. The pack is intended to simulate a Bowie-Dick testpack. However, the evacuation of air from and introduction of steam intothe stack will occur on all six sides of the test pack, and will tend tofollow the path of least resistance. Further, the stack of porous sheetsconsists of an inner "core" of sheets and an outer "shell" of sheets.The sheets constituting the "core" are of a porosity that is lower thanthose constituting the "shell." Therefore, air and steam travel willoccur to a greater extent through the inner "core," and the desiredeffect of the two different porosities is eliminated. Also, depending onthe smoothness and degree of contact among the sheets, within eachlayer, the flow of steam into the stack will vary, and, correspondingly,the indicator will give varying results. Finally, the degree of drynessof the paper forming the layers will affect the indication provided bythe stack.

Dyke and Oshlag U.S. Pat. No. 4,594,223 discloses an insulated heat sinkplaced in "series" with a chamber containing an indicator for detectingthe presence of-air within a steam sterilizer. The heat sink can befibrous or a metal coated with an insulator. As steam contacts the heatsink it gives up its latent heat, which cause the steam to change state,and accumulate any air mixed with the steam. Newer prevacuum steamsterilizers use a conditioning-evacuation cycle prior to sterilization,which replaces the original deep or continuous evacuation. This newercycle typically commences with a one minute steam purge with the drainopen followed by a vacuum pulse and three intermittent steam and vacuumpulses with the drain closed. Each pulse occurs at a prescribed pressureto condition the load and eliminate air prior to sterilization. Duringthe steam purge and pulses, the temperature of the heat sink rises, andits ability to function is compromised. Therefore, the heat sink cannoteffectively separate air from steam during the critical sterilizationcycle.

Accordingly, despite the efforts of workers in the art, there remains aneed for an effective indicator of the presence of air during steamsterilization that provides the benefits of a Bowie-Dick test pack.

SUMMARY OF THE INVENTION

All current Bowie and Dick type devices use a heat sink, which mustchange the state of steam to water to accumulate any air in the steam.The capacity of the heat sink is limited, and, thus, often is renderedineffective during the sterilization cycle to collapse steam. Thepresent invention accumulates any gas in the steam without the need fora heat sink: The present invention employs a material that colectivelyabsorbs water That is, the material adsorbs the steam but does notadsorb air present within the steam. The physical structure of thematerial, and not its ability to absorb heat, permits it to separate airfrom steam. The capacity of the material permits it to remain effectivewell into the sterilization cycle. Also, some water can be desorbed fromthe material by exposing it to a vacuum Thus, the capacity of thematerial is regenerated to some extent during pre-sterilizationevacuation cycles.

Therefore, the present invention can function to separate air from steamfurther into the sterilization cycle than other indicators. The presentinvention also retains the remaining benefits provided by a Bowie-Dicktest pack.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments can beunderstood better if reference is made to the attached drawing, inwhich:

FIG. 1 is a top plan view of the preferred indicator provided by thepresent invention;

FIG. 2 is a bottom view of the indicator shown in FIG.

FIG. 3 is a side sectional view of the indicator shown in FIG. 1;

FIG. 4 is an isometric, exploded view of a portion of the indicatorshown in FIG. 1;

FIG. 5 is a side, sectional view of an alternate indicator provided bythe present invention; and

FIG. 6 is a top plan view of another alternate indicator provided by thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By way of introduction, newer prevacuum steam sterilizers use anevacuation cycle that is actually a combination of conditioning andevacuation. This combination compromises the use of a heat sink in itsability to separate air from steam. During this cycle, typically a oneminute steam purge is initially conducted with the drain open. Then avacuum pulse and three additional steam and prevacuum pulses areconducted in a closed system. Each pulse reaches a critical pressure.The heat transferred from the initial steam purge and subsequent pulsesreduces the heat absorptive capabilities of a heat sink. Therefore, thepresent invention employs a material that does not rely on a temperaturedifference between the steam and a part of the indicator, usually a heatsink, to collapse steam to separate air from it. The present inventionemploys a material that physically adsorbs steam but not air, due to itsstructure. Thus, any heating of the material during any part of thecycle does not compromise its ability to function. In fact, due to thestructure of the material, the material has a capacity sufficient toseparate air from steam during pre-sterilization cycles and thesterilization cycle itself. Any regenerative capabilities of thematerial simply enhance its capacity. The material can be an adsorbent adesiccant, or any other suitable material.

For example, the metal alumino-silicate crystals that are employed bythe preferred embodiments of the present invention instead of a heatsink physically adsorb steam during each conditioning steam pulse. Thatsteam is desorbed to some extent during each vacuum pulse, therebyregenerating the adsorbent somewhat before the sterilization cycle iscommenced. This desorption actually is aided by the temperature increaseof the adsorbent during the process. Thus, unlike a heat sink, thisclass of materials can detect all steam-air problems since its utilitycan extend well into the sterilization portion of the cycle.

In the construction of devices 10, 100 and 200, which are shown in FIGS.1 through 6, steam is constrained to follow a prescribed path. Steamenters the device through a sealable opening into a receptaclecontaining an adsorbent. The receptacle is designed to provide maximumcontact between the steam and the adsorbent, and it can define atorturous path to further this purpose. At the distal end of thereceptacle is a small opening that provides communication between thereceptacle and an air collecting chamber. The chamber contains anindicator. The indicator can be a typical known biological or chemicalindicator, or it can be a combination indicator of known type. As iswell known in the art, a chemical indicator includes a steam sensitiveink, which changes color upon exposure to steam. As is also well knownin the art, a biological indicator includes microorganisms that will bedestroyed during steam sterilization. A variety of chemical andbiological indicators are commercially available.

The top section of the device, which seals the air collecting chamberand covers the indicator, can be peeled or broken away to retrieve theindicator for record purposes.

In use, a tab covering the entrance opening to the adsorbent receptacleis removed and the device is, typically, placed in the coolest sectionof the sterilizer, usually over the drain, where air is likely to bepresent. During the evacuation portion of the cycle, air is withdrawnfrom the interstices of the adsorbent receptacle and air collectingchamber. Entering the sterilization portion of the cycle, steam isforced into the adsorbent receptacle where it is selectively adsorbed bythe adsorbent material. Any air mixed with the steam is not adsorbed andis forced further through the receptacle in a progressive manner asareas of the adsorbent becomes saturated. When the adsorbent is totallysaturated, the air and steam forced into the air collecting chamber.Here, if air is present, it will be forced to the end distal from theopening to the receptacle, and it will shield the indicator, thuspreventing a uniform change that would show complete reaction withsteam. Upon completion of the sterilization cycle, the device iswithdrawn from the sterilizer and a determination of the effectivenessof the cycle can be made.

FIGS. 1 through 6 show the preferred embodiments of the indicatorprovided by the present invention. FIGS. 1 through 4 show an indicator10, which is typically placed within the chamber of a prevacuum typesteam sterilizer (not shown) for the purpose of detecting and indicatinginadequate air removal during evacuation, and introduction of air duringevacuation through a leak or during the sterilization cycle.

Device 10 includes a top 12, and a housing 18, which includes a section30 that is sealed along its perimeter to housing 18. The assembly ofhousing 18 and section 30 is sealed along the perimeter of housing 18 totop 12. Device 10 includes an indicator strip 14 located beneath top 12,and a grip section 16, which forms a part of top 12. Referring to FIG.2, housing 18 defines adsorbent receptacle 20 that is adapted to containa steam adsorbent 22. Housing 18 also defines a portal 24 which allowssteam to enter housing 18. A tab 26 is secured to the bottom of housing18 over portal 24. Tab 26 maintains the integrity of the interior ofhousing 18 and adsorbent 22 during shipping and storage of device 10.Section 28, which is defined by housing 18 cooperates with section 16 oftop 12 to provide a grip for device 10. Top 12, including section 16, isso secured to housing 18 as to permit a user to peel top 12 from housing18 to gain access to indicator 14. Indicator 14 is, for discussionpurposes, a chemical indicator. That is, indicator 14 includes steamsensitive ink.

Referring to FIG. 3, section 30 is sealed to housing 18 to enclosereceptacle 20. Section 30 also cooperates with top 12 to define an aircollection chamber 34, in which indicator 14 is located. Adsorbent 22can be loaded into receptacle 20 prior to securing section 30 in place.Alternately, adsorbent 22 can be loaded into receptacle 20 throughportal 24 after section 30 has been secured to housing 18. Aircollection chamber 34 is in communication with receptacle 20 through aportal 32, which is defined by section 30. Preferably, the componentsshown in FIG. 4 are assembled together by securing section 30 to housing18, placing indicator strip 14 on top of section 30, and securing top 12to housing 18.

The preferred adsorbent 22 for device 10 is crystalline metalalumino-silicates and, more specifically, Na₈₆ [(AlO₂)₈₆ (SiO₂)₁₀₆ ] XH₂ O, which has been activated for water adsorption by removing thewater of hydration by heating. Crystalline metal alumino silicates ofthe type preferred can be purchased from Union Carbide Corporation,Danbury, Conn. The crystal structure of the metal alumino-silicates is atruncated octahedra joined in a cubic array, or honeycomb structure,with relatively large cavities. Each cavity is connected with sixadjacent cavities through apertures. The efficiency of this material forthis application is the selective adsorption of water resulting fromwater's unique molecular size and polarity, along with the uniform sizeand molecular dimensions of the crystal's cavities, the extremely largesurface area resulting from the honeycomb, and the high capacity foradsorption over a wide range of operating conditions includingtemperatures to over 600° F. Additionally, the large volume of airwithin an aggregate of crystals offers a challenge to air removal duringthe evacuation portion of the steam sterilization cycle. The quantity ofadsorbent will vary with the application in a manner understood by thoseof ordinary skill in the art upon reference to manufacturers' publishedinformation. The sizing of housing 18 will depend on the quantity ofmaterial it must contain.

To use device 10, tab 26 is removed from the bottom of housing 18, anddevice 10 is placed in the chamber of a prevacuum steam sterilizer.During the newer pulsing prevacuum cycle, the initial steam purge forcessome steam through portal 24 and into receptacle 20, where it isadsorbed by adsorbent 22 located at proximal end 40 of receptacle 20.Following the initial steam purge, and each subsequent steam pulsecycle, during which steam is adsorbed by adsorbent 22, an evacuationpulse is produced. During each evacuation pulse, adsorbed moisture isdesorbed to some extent and exits receptacle 20 through portal 24. Thus,adsorbent 22 is regenerated to some extent during each evacuation pulse.The initial steam purge and each subsequent steam pulse occurring duringthe conditioning-evacuation portion of the cycle increase thetemperature of device 10, including absorbent 22. During the evacuationpulses, air exits adsorbent 22, through portal 24, and air collectingchamber 34, through portal 32. As steam enters the sterilizer chamber,it is forced into device 10 through portal 24. Upon contacting adsorbent22, the steam is adsorbed, leaving any air present in the steam, even ifthe adsorbent 22 is at the sterilization temperature. Thus, unlikedevices that rely on heat sinks to separate air from steam, the efficacyof device 10 extends throughout the sterilization cycle. As the steamfront progresses from proximal end 40 of receptacle 20 to distal end 42,it forces any accumulated air ahead of it through receptacle 20. Baffles36 and 38 formed in sections 18 and 30, respectively, define a tortuouspath for steam traveling through receptacle 20, and create turbulence inthe steam. Turbulence in the steam flow aids intimate contact betweenthe steam and adsorbent 22. Ultimately, the steam forces any accumulatedair through portal 32 and into air collecting housing 34. If air is notpresent in chamber 34, which means the sterilization cycle wassuccessful, the ink on chemical indicator will have changed coloruniformly. Otherwise, the color change will not be uniform.

FIGS. 5 and 6 show devices 100 and 200, which include alternativearrangements for providing access to air collecting chamber 34 andindicator strip 14. Elements shown in FIGS. 5 and 6 have been assignedthe reference characters of the corresponding elements shown in FIGS. 1through 4. The housing 18 of each of devices 100 and 200 is made from asuitable plastic material. As is well-known to those in the art, aplastic material will exhibit lower tear and crack resistance to forcesapplied along the molecular orientation of the plastic than to forcesapplied perpendicular to the molecular orientation. The molecularorientation of housing 18 of each of devices 100 and 200 should be 90°to the longitudinal axis of housing 18. The molecular orientation of top12 of device 200 should also be 90° to the longitidinal axis of device10.

A detente 46 is formed in the bottom surface of housing 18 of device 100across its width. Access to end 50 of strip 14 is gained by bending end48 of top 12 and tab 28 of housing 18 upward, relative to theorientation of device 100 in FIG. 5, thereby applying a focused stressalong detente 46, until a fracture occurs.

Top 12 of device 200 defines a pair of detentes 52. Detentes 52facilitate tearing end 16 away from housing 18 and the remainder of top12 to gain access to end 50 of strip 14.

The preferred method provided by the present invention employs devices10, 100, or 200 during sterilization to indicate the effectiveness ofthe sterilization cycle.

What is claimed is:
 1. A method for the detection of air in a closedsterilization system comprising the steps of:a. placing an airindicating device into a sterilization chamber; b. creating anatmosphere capable of sterilization in the chamber by alternatingintroduction of vacuum and steam to substantially remove air within saidchamber and fill said chamber with steam at sterilization conditions; c.said steam at sterilization conditions contacting the air indicatingdevice, the device which comprises a column containing an absorbent thatdirectly removes the steam by a process selected from the groupconsisting of absorption, adsorption and combinations thereof, without achange of state of the steam to a liquid liberating any airinadvertently mixed with the steam and an indicator for detecting thepresence of the liberated air; d. observing the indicating device todetermine whether there was air present within the steam in the chamber.2. The method of claim 1 wherein said test device includes a housingdefining a column and a quantity of absorbent contained therein, saidhousing defining a column in which steam is introduced into the proximalend of the column and is forced to travel progressively to the distalend of said column.
 3. The method of claim 2 wherein said column furtherincludes an adjacent chamber proximate its distal end for collection ofair, said chamber further including means for indicating the presence ofair concentrated in said chamber.
 4. The method of claim 3 wherein saidabsorbent is a metal alumino-silicate.
 5. The method of claim 4 whereinsaid metal alumino-silicate is Na₈₆ [(AlO₂ ]₈₆ (SiO₂)₁₀₆ ].XH₂ O, with aportion of the water of hydration being removed prior to use.
 6. Adevice for the detection of air in a closed sterilization systemcomprising:a. an air indicating device placed into a sterilizationchamber; b. means for creating an atmosphere capable of sterilization insaid chamber by alternating introduction of vacuum and steam pulses tosubstantially remove air within said chamber and fill said chamber withsteam at sterilization conditions; c. means for passing said steam atsterilization conditions in contact with the air indicating device, thedevice which comprise a column containing an absorbent that directlyremoves the steam by a process selected from the group consisting ofabsorption, adsorption and combinations thereof, without a change ofstate of tire steam to a liquid liberating any air inadvertently mixedwith the steam and an indicator for detecting the presence of theliberated air.
 7. The device of claim 6 wherein said test deviceincludes a housing defining a column and a quantity of absorbentcontained therein said housing defining a column in which steam isintroduced into the proximal end of tire column and is forced to travelprogressively to the distal end of said column.
 8. The device of claim 5wherein said column further includes an adjacent chamber proximate itsdistal end for collection of air said chamber further including meansfor indicating the presence of air concentrated in said chamber.
 9. Thedevice of claim 8 wherein said absorbent is a metal alumino-silicate.10. The device of claim 9 wherein said metal alumino-silicate is Na₈₆[(AlO₂ ]₈₆ (SiO₂)₁₀₆ ].XH₂ O, with a portion of the water of hydrationbeing removed prior to use.